A heat pipe can conduct heat from a heat source such as from an electronic device through vapor heat transfer. Typically, the heat pipe includes a working fluid, an evaporator section, and a condenser section. The working fluid is vaporized at the evaporator section. The vapor is received at the condenser section, whereupon the vapor is condensed to form a liquid working fluid. Capillary action returns the condensed working fluid to the evaporator section, thereby completing a cycle.
In a variable-conductance heat pipe (VCHP), the conductance of the heat pipe will vary depending on the operating temperature. This is typically achieved through a non-condensable gas, e.g., a noble gas such as helium, argon, nitrogen or the like, in an interior of the heat pipe. The non-condensable gas resides in passages adjacent to the condenser section. As the heat load from a heat source increases or as the evaporator temperature increases, the vapor pressure of the working fluid increases, forcing the non-condensable gas to compress and expose more of the condenser area. The dense vapor of the working fluid can then reach the exposed condenser surface for vapor condensation. On the other hand, when the evaporator is at a low temperature, the volume of the non-condensable gas increases, thereby increasing the blocked part of the condenser. The working fluid has a low vapor pressure allowing the component to warm up before the heat is removed. Due to the low vapor pressure, a relatively high volumetric flow rate would be needed to achieve a given amount of heat transfer. This high vapor flow rate can in turn facilitate maintaining the heat source at a relatively constant temperature despite a variation in the heat pipe's operating temperature.